Salmonellosis: Introduction - The Merck Veterinary Manual

Salmonellosis: Introduction

Etiology, Epidemiology, and Pathogenesis

Cattle and Sheep

Control and Prevention

Prevention of Introduction

Limitation of Spread Within a Herd

Salmonella Vaccines

Salmonellosis in Veterinary Teaching Hospitals

Salmonellosis is caused by many species of salmonellae and characterized clinically by one or more of 3 major syndromes—septicemia, acute enteritis, and chronic enteritis. The disease is seen worldwide and in all animals. The incidence has increased with the intensification of livestock production. Young calves, piglets, lambs, and foals usually develop the septicemic form (see diarrhea in neonatal ruminants, Diarrhea in Neonatal Ruminants, and diarrheal diseases of foals, Intestinal Diseases in Ruminants). Adult cattle, sheep, and horses commonly develop acute enteritis, and chronic enteritis may develop in growing pigs and occasionally in cattle (see also the chapters on intestinal diseases in each of the major domestic species, The Exocrine Pancreas: Introduction et seq). Pregnant animals may abort. The clinically normal carrier animal is a serious problem in all host species. Salmonellosis is seen infrequently in dogs and cats and is characterized by acute diarrhea with or without septicemia. The incidence of human salmonellosis has increased in recent years, and animals have been incriminated as the principal reservoir. Transmission to humans occurs via contaminated drinking water, milk, meat, and foods such as cake mixes that use contaminated ingredients; poultry and eggs ( Salmonelloses: Introduction) are particularly important sources of infection.

Etiology, Epidemiology, and Pathogenesis:

While many other Salmonella spp may cause disease, the more common ones in each species are as follows: Cattle— Salmonella serovar typhimurium , Salmonella serovar dublin , and Salmonella serovar newport ; Sheep and goats— S typhimurium , S dublin , Salmonella serovar anatum , and Salmonella serovar montevideo ; Pigs— S typhimurium and Salmonella serovar choleraesuis ; Horses— S typhimurium , S anatum , S newport , Salmonella serovar enteritidis , and Salmonella serovar IIIa 18:z₄z₂₃. Although their resulting clinical patterns are not distinct, different species of salmonellae tend to differ in their epidemiology. Plasmid profile and drug-resistance patterns are sometimes useful markers for epidemiologic studies. Feces of infected animals can contaminate feed and water, milk, fresh and processed meats from abattoirs, plant and animal products used as fertilizers or feedstuffs, pasture and rangeland, and many inert materials. The organisms may survive for months in wet, warm areas such as in feeder pig barns or in water dugouts but survive <1 wk in composted cattle manure. Rodents and wild birds also are sources of infection. Pelleting of feeds reduces the level of contamination by salmonellae. The prevalence of infection varies among species and countries and is much higher than the incidence of clinical disease, which is commonly precipitated by stressful situations such as sudden deprivation of feed, transportation, drought, crowding, parturition, and the administration of some drugs. Salmonellosis is common in hospitalized horses that have been subjected to prolonged surgical procedures. Use of oral antimicrobial agents is sometimes a risk factor for the disease.

The usual route of infection is oral and, after infection, the organism multiplies in the intestine and causes enteritis. Greater susceptibility of the young may be due to high gastric pH, absence of a stable intestinal flora, and limited immunity. Penetration of bacteria into the lamina propria likely contributes to gut damage and diarrhea. The inflammatory response is marked, and salmonellae are engulfed by phagocytic cells; however, the bacteria can survive and multiply in these cells. Septicemia may follow with subsequent localization in brain and meninges, pregnant uterus, distal aspects of the limbs, and tips of the ears and tails, which can result, respectively, in meningoencephalitis, abortion, osteitis, and dry gangrene of the feet, tail, or ears. The organism also frequently localizes in the gallbladder and mesenteric lymph nodes, and survivors intermittently shed the organism in the feces.

Calves rarely become carriers but virtually all adults do for variable periods—up to 10 wk in sheep and cattle and up to 14 mo in horses. Adult cattle infected with S dublin excrete the organism for years. Infection may persist in lymph nodes or tonsils, with no salmonellae in the feces. Latent carriers may begin shedding the organism or even develop clinical disease under stress. A passive carrier acquires infection from the environment but is not invaded, so that if removed from the environment, it ceases to be a carrier.

Cattle and Sheep:

In calves and lambs, the disease is usually endemic on a particular farm, with sporadic explosive outbreaks. Subclinical infection with occasional herd outbreaks may be seen in adult cattle. Stressors that precipitate clinical disease include deprivation of feed and water, minimal levels of nutrition, long transport times, calving, and mixing and crowding in feedlots.

Pigs:

Outbreaks of septicemic salmonellosis in pigs are rare and usually can be traced to a purchased, infected pig. Purchase of feeder pigs from salmonellae-free herds and use of the “all-in/all-out” policy in finishing units minimize exposure.

Horses:

Many horses may be carriers. In adults, most cases develop after the stress of surgery or transport, especially when horses are moved through sales yards, deprived of feed and water, and then overfed at their destination. Mares may be inapparent shedders and, despite several negative cultures before foaling, may shed the bacteria at parturition and infect the newborn foal. Salmonellosis in horses hospitalized for other causes is a major problem for equine clinics and stud farms. In these circumstances, carriers are constantly reintroduced, the environment is persistently contaminated, and a large population of vulnerable horses is at risk. Septicemic salmonellosis is also common in foals; it may be endemic on a given premises or there may be outbreaks. (See also intestinal diseases in horses and foals, Intestinal Diseases in Horses and Foals: Introduction.)

Dogs and Cats:

Many dogs and cats are asymptomatic carriers of salmonellae. Clinical disease is uncommon, but when it is seen, it is often associated with hospitalization, another infection or debilitating condition in adults, or exposure to large numbers of the bacteria in puppies and kittens.

Clinical Findings:

Septicemia is the usual syndrome in newborn calves, lambs, foals, and piglets, and outbreaks may occur in pigs up to 6 mo old. Illness is acute, depression is marked, fever (105-107°F [40.5-41.5°C]) is usual, and death occurs in 24-48 hr. In pigs, a dark red to purple discoloration of the skin is common, especially of the ears and ventral abdomen. Nervous signs may be seen in calves and pigs; these animals may also suffer from pneumonia. Mortality may reach 100%.

Acute enteritis is the common form in adults as well as in calves that are usually ≥1 wk old. Initially, there is fever (105-107°F [40.5-41.5°C]), followed by severe watery diarrhea, sometimes dysentery, and often tenesmus. In a herd outbreak, several hours may lapse before the onset of diarrhea, at which time the fever may disappear. The feces, which vary considerably, may have a putrid odor and contain mucus, fibrinous casts, shreds of mucous membrane, and in some cases, large blood clots. Rectal examination causes severe discomfort, tenesmus, and commonly dysentery. Milk production often declines precipitously in dairy cows. Abdominal pain is common and severe in horses. Affected horses are severely dehydrated and may die within 24 hr of the onset of diarrhea; mortality may reach 100%. A marked leukopenia and neutropenia are characteristic of the acute disease in horses. In dogs and cats, clinical disease takes the form of acute diarrhea with septicemia and is seen occasionally in puppies and kittens or in adults stressed by concurrent disease. Pneumonia may be evident. Abortion is likely to occur in pregnant bitches or queens. Conjunctivitis is sometimes seen in affected cats.

Subacute enteritis may develop in adult horses and sheep on farms where the disease is endemic. The signs include mild fever (103-104°F [39-40°C]), soft feces, inappetence, and some dehydration. There may be a high incidence of abortion in cows and ewes, some deaths in ewes after abortion, and a high mortality rate due to enteritis in lambs younger than a few weeks of age. In cattle, the first signs may be fever and abortion, followed several days later by diarrhea.

Chronic enteritis is a common form in pigs and adult cattle. There is persistent diarrhea, severe emaciation, intermittent fever, and poor response to treatment. The feces are scant and may be normal or contain mucus, casts, or blood. In growing pigs, rectal stricture may be a sequela if the terminal part of the rectum is involved. Affected pigs are anorectic and lose weight; the abdomen becomes grossly distended. The stricture is obvious on digital palpation and necropsy.

A number of Salmonella spp are found in foxes, especially kits, and produce a peracute enteritis. Fur-bearing and zoo carnivores may be affected. Contaminated feed is often the source of infection. Several rodents (eg, guinea pigs, hamsters, rats, and mice) and rabbits are susceptible. Rodents commonly act as a source of infection on farms where the disease is endemic. Pet turtles were once a common source of infection in humans that has been virtually eliminated by the curtailment of commercial trafficking in turtles.

Diagnosis:

This depends on the clinical signs and on the laboratory examination of feces, tissues from affected animals, feed (including all mineral supplements used), water supplies, and feces from wild rodents and birds that may inhabit the premises. The clinical syndromes usually are characteristic but must be differentiated from several similar diseases in each species as follows: Cattle—diarrhea due to enterotoxigenic Escherichia coli , dysentery due to verotoxigenic E coli , coccidiosis, cryptosporidiosis, the alimentary tract form of infectious bovine rhinotracheitis, bovine viral diarrhea, hemorrhagic enteritis due to Clostridium perfringens types B and C, arsenic poisoning, secondary copper deficiency (molybdenosis), winter dysentery, paratuberculosis, ostertagiasis, and dietetic diarrhea; Sheep—enteric colibacillosis, septicemia due to Haemophilus sp or pasteurellae, and coccidiosis; Pigs—enteric colibacillosis of newborn pigs and weanlings, swine dysentery, campylobacteriosis, and the septicemias of growing pigs (which include erysipelas, classical swine fever, and pasteurellosis); Horses—septicemia (due to E coli , Actinobacillus equuli , or streptococci) and colitis-X.

Salmonellosis, equine colon

The lesions are those of a septicemia or a necrotizing fibrinous enteritis, or both. Lesions are most severe in the lower ileum and the large intestine and vary from shortening of villi with loss of the epithelium to complete loss of intestinal architecture. There is a neutrophilic reaction in the lamina propria, and thrombi may be seen in blood vessels in this region. Hemorrhage and fibrin strands are usually seen. Culture techniques that involve suppression of fecal E coli are usually necessary, and several daily fecal cultures may be necessary to isolate the organism. Blood cultures in septicemic animals may be rewarding but are costly. Serologic testing is difficult to interpret.

Treatment:

Early treatment is essential for septicemic salmonellosis, but there is controversy regarding the use of antimicrobial agents for intestinal salmonellosis. Oral antibiotics may deleteriously alter the intestinal microflora, interfere with competitive antagonism, and prolong shedding of the organism. There is also concern that antibiotic-resistant strains of salmonellae selected by oral antibiotics may subsequently infect humans.

Broad-spectrum antibiotics are used parenterally to treat the septicemia. Initial antimicrobial therapy should be based on knowledge of the drug resistance pattern found in the area. Nosocomial infections often involve highly drug-resistant organisms. Trimethoprim-sulfonamide combinations are often effective. Alternatives are ampicillin, fluoroquinolones, or third-generation cephalosporins. Treatment should be continued daily for up to 6 days. Oral medication should be given in drinking water because affected animals are thirsty due to dehydration, and their appetite is generally poor. Fluid therapy to correct acid-base imbalance and dehydration is necessary. Calves, adult cattle, and horses need large quantities of fluids. Antibiotics such as ampicillin or cephalosporins lead to lysis of the bacteria with release of endotoxin. NSAID may be used to reduce the effects of endotoxemia. Horses with acute intestinal salmonellosis are severely acidotic and hyponatremic and may need to be treated initially with 5% sodium bicarbonate, IV, at 5-8 L/450 kg body wt. This is followed by balanced electrolytes containing potassium to correct the hypokalemia that may follow correction of the acidosis. In horses, flunixin meglumine is recommended for its antiendotoxic properties. Corticosteroids are not recommended because of their immunosuppressive effects and their potential to exacerbate laminitis. IV administration of plasma with a high titer of Salmonella lipopolysaccharide core antibodies may be beneficial in horses. Plasma with serotype-specific antibodies is even more beneficial. Septicemic salmonellosis in pigs usually responds favorably if treated early. However, the intestinal form is difficult to treat effectively in all species. Although clinical cure may be achieved, bacteriologic cure is difficult, particularly in adult animals, because the organisms become established in the biliary system and are intermittently shed into the intestinal lumen, which causes chronic relapsing enteritis and contamination of the environment.

Control and Prevention:

These are major problems because of carrier animals and contaminated feedstuffs. Drain swabs or milk filters may be cultured to monitor the salmonellae status of a herd. The principles of control include prevention of introduction and limitation of spread within a herd.

Prevention of Introduction:

Every effort must be made to prevent introduction of a carrier; animals should be purchased directly only from farms known to be free of the disease and should be isolated for ≥1 wk while their health status is monitored. Ensuring that feed supplies are free of salmonellae depends on the integrity of the source.

Limitation of Spread Within a Herd:

In an outbreak, the following procedures should be implemented: 1) Carrier animals should be identified and either culled or isolated and treated vigorously. Treated animals must be rechecked several times before there can be confidence that they are not carriers. 2) The prophylactic use of antibiotics in feed or water supplies may be considered (but the hazards have been mentioned above). 3) Movement of animals around the farm should be restricted to limit infection to the smallest group. Random mixing of animals should be avoided. 4) Feed and water supplies must be protected from fecal contamination. 5) Contaminated buildings must be vigorously cleaned and disinfected. 6) Contaminated material must be disposed of carefully. 7) All persons should be aware of the hazards of working with infected animals and the importance of personal hygiene. 8) Use of a vaccine should be considered, particularly in an outbreak involving pregnant cattle in which a vaccine has been shown to confer some protection in adults and calves. Commercial killed bacterins or autogenous bacterins may be used. Live attenuated vaccines show considerable promise, but they are not available commercially. 9) Stresses should be minimized. 10) Fetal membranes of mares should be placed in a plastic bag until the mare eliminates them, and the foal should be fed colostrum before contact with the mare.

Salmonella Vaccines:

Salmonellae are intracellular parasites, and a live vaccine is therefore expected to be necessary for optimal immune protection against disease. Several studies with live attenuated Salmonella vaccines in pigs, cattle, and chickens have shown them to be effective in stimulating a strong cell-mediated immune response and protecting animals against disease. A live attenuated S choleraesuis vaccine that has been licensed for use in swine appears to be effective in reducing colonization of tissues and protecting pigs from disease following challenge with virulent organisms. This vaccine has also been reported to reduce the prevalence of Salmonella infections in pigs under field conditions. This vaccine also protected calves against experimental challenge with S Dublin following intranasal or subcutaneous administration of the vaccine. The administration of the S choleraesuis vaccine to pregnant dairy cows reduced the frequency of shedding of serogroup C1 salmonellae during the peripartum period. Live S enteritidis has been effective in significantly reducing the infection of laying hens challenged with S enteritidis . The commercial vaccines available for use in cattle are bacterins and appear to induce a modest level of protection. When given to pregnant cows, they induce antibodies in colostrum, which provide an important measure of protection to calves, which are most susceptible in the first week of life.

Salmonellosis in Veterinary Teaching Hospitals:

The risk of equine salmonellosis epidemics is particularly high in veterinary teaching hospitals. A high case load, the presence of animals with severe illnesses, and the stresses of transportation contribute to this risk. The serotype most commonly implicated is S typhimurium but other serotypes that have been identified include Salmonella serovar heidelberg , Salmonella serovar krefeld , Salmonella serovar infantis , and S anatum . They are frequently resistant to multiple drugs. Contamination of the hospital environment often begins with the introduction of the organisms by a carrier animal. Animals in the hospital (which are often highly susceptible) readily develop diarrhea and contribute to further contamination of the hospital. Infection rates as high as 18% have been reported. Nosocomial infection can be identified by development of salmonellosis ≥3 days after hospitalization and by isolation of hospital-associated Salmonella from cases. The identification of hospital-associated Salmonella involves determination of serotype, drug resistance patterns, and possibly other strain markers such as pulsed field gel electrophoresis patterns, ribotype, and plasmid profiles. Many hospitals have developed protocols designed to minimize the occurrence of these epidemics, including isolating horses with diarrhea, controlling animal traffic patterns, routinely disinfecting the hospital with an agent of known effectiveness, controlling rodents, using footbaths, limiting access to the hospital area, regularly monitoring drains and other areas of the hospital for salmonellae, routinely repeatedly monitoring horses admitted to the hospital to determine whether they are shedding Salmonella , wearing barrier clothing when attending animals at high risk, and educating personnel. Renovations to the hospital may be required to facilitate thorough cleaning. Closing the hospital for a period of time is often necessary to thoroughly clean and disinfect the premises and to break the cycle of transmission. Outbreaks of salmonellosis have been reported among dogs in veterinary teaching hospitals, but these occurrences are uncommon; antibacterial therapy is a predisposing factor.